Spin Hall effect (SHE), particularly giant spin Hall effect (GSHE), in non-magnetic metals with strong spin-orbit coupling (SOC), has received much attention in development and improvement of MRAM and SLD. See, Dyakonovet et al., Phys. Lett. A35, 459 (1971); Hirsch, Phys. Rev. Lett. 83, 1834 (1999); Azevedo et al., Phys. Rev. B 83, 144402 (2011); Vlietstra et al., Appl. Phys. Lett. 103, 032401 (2013); Liu et al., Phys. Rev. Lett. 106, 036601 (2011); Lee et al., Phys. Rev. B 89, 024418 (2014); Ganguly et al., Appl. Phys. Lett. 104, 072405 (2014); Liu et al., Science 336, 555 (2012); Pai et al., Appl. Phys. Lett. 104, 082407 (2014); Pai et al., Appl. Phys. Lett. 101, 122404 (2012); Mellnik et al., Nature 511, 449 (2014); Fan et al., Nature Materials 13, 699 (2014); Zhang, Phys. Rev. Lett. 85, 393 (2000). Large spin Hall angles (θ) have been discovered in solids ranging from the simple SOC solids: platinum (θ=0.08), tantalum (θ=0.15), and tungsten (θ=0.30) to topological insulators: Bi2Se3 (θ=2.0-3.5) and BiSbTe3. See, Azevedo et al., Phys. Rev. B 83, 144402 (2011); Vlietstra et al., Appl. Phys. Lett. 103, 032401 (2013); Liu et al., Phys. Rev. Lett. 106, 036601 (2011); Lee et al., Phys. Rev. B 89, 024418 (2014); Ganguly et al., Appl. Phys. Lett. 104, 072405 (2014); Pai et al., Appl. Phys. Lett. 104, 082407 (2014); Pai et al., Appl. Phys. Lett 101, 122404 (2012); Mellnik et al., Nature 511, 449 (2014); Fan et al., Nature Materials 13, 699 (2014); Liu et al., Science 336, 555 (2012). The spin Hall angle is used to manipulate magnetization states in spintronic devices. See, Liu et al., Phys. Rev. Lett. 106, 036601 (2011); Liu et al., Science 336, 555 (2012); Fan et al., Nature Materials 13, 699 (2014). A material with a larger spin Hall angle is more efficient at converting longitudinal electrical charge current (Jc) to a transverse spin current (Js) than a material with a smaller spin Hall angle.
Tungsten (W) has the largest spin Hall angle (0.3) among transition metals. The preparation of W has been developed to be compatible with modern semiconductor fabrication processes. W metallization processing is widely used for very-large-scale-integration (VLSI) circuits, and the stable and conductive α-W is more commonly used in semiconductor processing than β-W. See, Petroff et al., Appl. Phys. Lett. 44, 2545 (1973). Structures of β-W/ferromagnetic thin film (FM) resulting in perpendicular magnetic anisotropy (PMA) have not previously been obtained. For MRAM and SLD applications, PMA is required for high performance. The common practice of inserting a hafnium layer (e.g. β-W/Hf/FM) to encourage PMA has the deleterious effects of lowering the effective spin Hall angle and increasing fabrication complexity. See, Pai et al., Appl. Phys. Lett. 104, 082407 (2014).
β-W has not been well studied in the context of GSHE, for example, the intrinsic spin Hall angle and spin diffusion length in bulk β-W previously remained undefined. The spin Hall angle dependency on β-W thin film thickness has not previously been measured and studied. Further, much effort has previously been spent to stabilize β-W. There is a need to develop methods of fabrication of stable β-W thin films that have a broad range of thicknesses and which simultaneously exhibit GSHE.